This invention relates to cutting elements for rotary drill bits for drilling or coring holes in subsurface formations. It relates in particular to drill bits which comprise a bit body having a shank for connection to a drill string, an operating end face on which are mounted a plurality of so-called "preform" cutting elements, and a passage for supplying drilling fluid to the operating face of the bit for cooling and/or cleaning the cutting elements.
Each preform cutting element comprises a thin layer of polycrystalline diamond or other superhard material bonded to a less hard backing layer, usually tungsten carbide. The two-layer arrangement of the cutting element provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer. The preform cutting elements are usually mounted on the bit body by being bonded, for example by brazing, to a carrier, which may be in the form of a stud of tungsten carbide which is secured in a socket in the bit body.
The preforms are normally manufactured by hot pressing a layer of polycrystalline diamond material and a layer of tungsten carbide in a high pressure, high temperature press, usually with a cobalt catalyst. Methods of manufacturing such preforms are described, for example, in U.S. Pat. No. 4,259,090.
Such preform cutting elements are commonly in the form of circular discs, and further details of bits using such cutting elements can, for example, be seen from the U.S. Pat. No. 4,505,342.
However, all cutting elements of this type will, in use, eventually become ineffective as a result of wear, and a bit will have to be taken out of service after an unacceptable amount of wear has occurred. Normally, this point is reached long before half of each circular disc cutting element has worn away. In view of this, therefore, the cost of manufacturing the drill bits is sometimes reduced by using cutting elements which do not take the form of a complete circular disc, but which take the form of a semi-circular or half disc. Obviously, two such cutting elements can be obtained simply by cutting a conventional circular cutting element in half. These half disc cutting elements can be mounted on carriers, or directly on the bit body, in much the same fashion as circular cutting elements, and usually the curved edge of each semi-circular element will form the cutting edge and engage the formation in use.
However, when a half disc cutting element is mounted on a carrier, such as a stud, the element forms a sharp, right angled internal corner with the surface of the carrier on which it is mounted, usually by brazing. There appears, in use, to be a stress concentration in this area with the result that the carrier tends to fracture and fail as a result of cracking which originates in this area and then propagates through the thickness of the carrier.
Another problem is that the use of a half disc cutting element reduces the area of the surface of the element which is brazed to the carrier or the bit body. This reduces the shear strength of the bond, making it more susceptible to failure, and detachment of the cutting element.
One of the objects of the present invention is to provide a preform cutting element, and a method of manufacturing such an element, which can maintain the cost advantages given by half disc cutting elements, but which may overcome the problems described above.
Although, as previously mentioned, preform cutting elements of the kind to which the present invention relates usually comprise two layers, the invention is also applicable to preforms comprising more than two layers. In particular, co-pending British Patent Appln. No. 88-7875-3, from which the present application claims priority, describes cutting elements in which a further front layer of less hard material, usually again tungsten carbide, is bonded to the front face of the diamond layer and extends across at least the major part thereof. Since the less hard material of the further layer may have better toughness in tension than the diamond layer, this may enable the cutting element better to resist tensile stress and thus allow a more positive back-rake of the diamond layer to be used.
The present invention also provides a method of manufacturing preform cutting elements. Such method is suitable for the manufacture of cutting elements to overcome the cracking problems referred to above, but it is found that the method is also suitable for convenient manufacture of certain of the multi-layer preform cutting elements described in the aforesaid co-pending patent application.